1. Field of the Invention
The present invention relates to a data transmission facility comprising a central station and substations respectively containing a data transmitter and a data receiver, in which the substations are connected to the central station by way of a data bus, in which, of the central station and the substations, only one transmits at one and the same time and the central station and/or the substations respectively comprise a control signal terminal which outputs a first control signal given a transmitting central station or, respectively, substation and outputs a second control signal given a central station or, respectively, a substation which is ready to receive.
2. Description of the Prior Art
A data transmission facility of the type set forth above is known from the German allowed and published application No. 22 51 608. In this known data transmission facility, modems of the stations output voice frequency pulses to a bus line.
The known data transmission facility is a matter of a remote control facility. When the remote control station wishes to output a pulse message or, respectively, a data protocol, it outputs a control signal already present in the remote control facility for characterizing a transmission authorization or readiness to receive, this control signal, among other things, activating or deactivating a switching amplifier which follows the transmission modulator of the modem. The switching amplifier and a further switching amplifier enable a common transmission and reception filter to be employed for the modem. The known station can be connected to a four-wire bus line for half-duplex operation. A coupling device is required for this purpose.
Coupling devices for coupling the modem of the remote control station to a four-wire line are disclosed in the German allowed and published application No. 20 48 140. In one embodiment, a plurality of hybrids are employed, while in another embodiment the coupling device is composed of a plurality of feedback-free transmission quadripoles.
Further, transmitters and receivers for data signals are already known which output or, respectively, receive direct voltage or direct current pulses. Asymmetrical and symmetrical interfaces for point-to-point connections or multiple connections are therefore known. A number of serial interfaces only permit point-to-point connections since only one driver can be connected to a line. This is the case, for example, given the wide-spread interfaces according to the EIA standards RS 232 C, RS-423 A, RS-422 A and the corresponding CCITT interfaces V24/V28, V10, V11, X26, X27. These interfaces are known, for example, from the publication CCITT, Yellow Book, Vol. VIII, fascicle VIII. 1, pp. 27-42, 43-55, 97-112, 161-165 and fascicle VIII. 2, pp. 190. This is insufficient for many applications.
The publication Elektronik No. 6/23.3.1984, p. 24, also discloses a serial interface according to EIA standard RS 485 which permits bus connections to up to 32 drivers.
However, there are numerous control and evaluation computers which are provided with a serial point-to-point interface and are suitable as a central station or, respectively, a routing central station of a data remote transmission facility.